Surveying, including electromagnetic surveying or seismic surveying, is used to perform characterization of subterranean elements in a subterranean structure. Examples of subterranean elements of interest include hydrocarbon-bearing reservoirs, fresh water aquifers, gas injection zones, and other subterranean elements. Seismic surveying is performed by deploying seismic sources (e.g., air guns, vibrators, explosives, etc.) and seismic receivers (e.g., geophones, hydrophones, etc.). The seismic sources are used to produce seismic waves that are propagated into the subterranean structure, with some of the seismic waves reflected from subterranean elements of interest. The reflected seismic waves are received by the seismic receivers.
Similarly, electromagnetic (EM) surveying can use EM sources and receivers. One type of EM surveying is referred to as controlled source EM surveying (CSEM), in which an EM transmitter is used to generate EM signals that are propagated into the subterranean structure. Subterranean elements reflect the EM signals, with the reflected EM signals received by the EM receivers.
In a marine survey environment, survey hardware components (e.g., sources and receivers) can be towed through a body of water. Typically, during acquisition of survey data, limits can be set for acceptable levels of positioning precision/accuracy measures. If any of the limits are exceeded (meaning that positioning quality measures cannot achieve the desired limits), then troubleshooting can be performed, which may involve reprocessing or even re-acquisition of survey data. This typically involves manual analysis in which an operator attempts to drill down into various information to find the source(s) of the problem.
In some conventional techniques, quality control (QC) statistics and other quality measures regarding positioning of survey hardware equipment can be delivered to a user on and/or off line, after acquisition of the survey data. The statistics and other quality measures can then be analyzed to determine if improper positioning of survey components may result in poorly acquired survey data. However, an issue associated with such analysis of QC statistics and measures regarding positioning precision/accuracy is that, if the statistics and measures do not show agreement with the expected values, then it may be necessary to re-acquire the survey data, which is very expensive.
Further, the expectations may not be realistic, causing confusion and extra work for both navigators and client QC representatives with respect to whether the position quality is adequate. In the worst case, the survey line(s) may be reacquired unnecessarily due to a poor understanding of what should be achievable.
In addition, any delay in validation of position estimates results in a delay in producing a fast track image, a preliminary unrefined image of the reservoir that is the target of the survey.
With the advent of full streamer acoustic networks, the number of acoustic measurements has grown to the point that even experts in adjustment computations are overwhelmed by the statistical analysis used in previous conventional networks including a few tens of measures. Full streamer acoustic networks include from a few hundred to thousands of acoustic ranges, often above 10,000 ranges in the case of IRMA networks. Such networks are practically impossible to troubleshoot for the typical navigator onboard today's seismic vessels. Another factor that aggravates this situation is the recent growth in seismic activity. This growth has resulted in an acute lack of expertise throughout the industry.
Because of the complexity of full streamer acoustic networks, the typical planner, both pre-survey and during the survey, cannot easily determine the optimum set of measurements in the context of typical failure situations. Due to an inability to visualize what to do, the normal approach to ensuring success is along the lines of more is better. Planners often deploy the wrong amount of equipment, either too much of a non-critical type or too little of the most critical type, because it is practically impossible to know the sensitivity of a particular spread to all measurements that could be made available. In some cases planners or ship personnel will attempt to meet a practically impossible specification by deploying as much equipment as possible. Only after the data is acquired and analyzed does it become clear that the goal was not possible without economically impractical means, such as additional vessels. What is not obvious is changes in survey accuracy with the shape of a spread. With the advent of full streamer acoustic networks, in particular IRMA networks, the seismic industry realized that the shape of the spread was a major factor in the positioning accuracy possible. Long skinny spreads have a weakness in the mid streamer area, furthest from the GPS control points. Spreads with shorter streamers and/or larger streamer separations become squarer, and thus have an improvement in the geometric distribution of positioning information.